A wide variety of high-molecular weight substances have hitherto been produced and used. High-molecular weight substances as produced (hereinafter referred to as crude high-molecular weight substances) are generally subjected to purification treatments to remove impurities contained therein.
Included in the impurities present in crude high-molecular weight substances are alkali metal compounds. Incorporation of an alkali metal compound into a high-molecular weight substance takes place in the following cases.
(1) In the production of oxyalkylene polymers by addition polymerization of an alkylene oxide (e.g., polypropylene oxide) or in the production of epoxy resins by reaction between bisphenol A and epichlorohydrin (e.g., bisphenol A-epichlorohydrin resin), an alkali metal compound used as a catalyst, e.g., sodium hydroxide and potassium hydroxide, remains in the produced high-molecular weight substance as it is or in the form of its salt. PA1 (2) In the separation of high-molecular weight substances produced by emulsion polymerization, an alkali metal compound used for salting out is incorporated into the resulting high-molecular weight substance. PA1 (3) In the reaction between a polymer containing a functional group, such as a oxyalkylene polymer containing a hydroxyl group or a carboxyl group or a diene polymer, and a halogen-containing compound, such as epichlorohydrin and allyl chloride, to produce a polymer having other functional group(s), an alkali metal compound used as a catalyst, e.g., sodium hydroxide and potassium hydroxide, remains in the resulting high-molecular weight substance as it is or in the form of its salt.
Besides the above-mentioned cases, there are many high-molecular weight substances having incorporated therein an alkali metal compound.
It is desirable that such an alkali metal compound remaining in high-molecular weight substances should be removed as sufficiently as possible to avoid possible adverse effects. For example, high-molecular weight substances for use as electronic parts should be purified to an extremely low alkali metal compound concentration since the existence of such an impurity causes reduction in electrical characteristics. Further, where a high-molecular weight substance is subjected to further reactions, the existence of an alkali metal compound sometimes adversely affects the reaction rate or yield.
Conventional techniques for removing an alkali metal compound from a high-molecular weight substance include (1) a treatment with an adsorbent, (2) a method comprising neutralizing the alkali metal compound with various acids to convert it to a different alkali metal compound, followed by filtration, and (3) extraction with water.
The method of using an adsorbent is unsuitable for removing a large quantity of an alkali metal compound. Moreover, there is only a limited choice in the kind of adsorbents which can be used for removal of alkali metal compounds in the form of a salt.
The method utilizing filtration requires growth of crystals of the neutralized salt. Should water be present, the salt would be dissolved.
From these and other reasons, the extraction method is preferred in many cases to the other two methods.
The extraction method comprises thoroughly contacting a crude high-molecular weight substance with water to transfer an alkali metal compound to an aqueous phase and separating the high-molecular weight substance from the aqueous phase. According to this method, the system is apt to be emulsified due to the presence of the high-molecular weight substance when subjected to, for example, vigorous agitation in order to obtain sufficient contact between the crude high-molecular weight substance and water. As a result, separation of the high-molecular weight substance from water takes much time, or large-sized equipment is required for separation. If the system is mildly agitated to prevent emulsification, extraction of the alkali metal compound would be inadequate. Apparatus which is employed in general extraction methods includes spray tower, a packed column, a column of a multistage type, an agitation type or a reciprocating motion type, a continuous counter-current extraction column, e.g., a pulse column, and a centrifugal extractor. Separation of an alkali metal compound from a crude high-molecular weight substance by the use of these apparatus requires a very long time or only results in insufficient extraction of the alkali metal compound.